Basic refractory

ABSTRACT

A fired periclase refractory of enhanced density and volume stability and good high temperature strength and thermal shock resistance is made from a batch comprising relatively coarse aggregate having 97.5 or more per cent MgO and, as matrix material, a prereacted grain containing substantial amounts of lime and silica, the ratio of lime to silica being at least sufficient to form dicalcium silicate.

0 United States Patent [1 1 [111 3,715,222

Hieb 1 Feb. 6, 1973 BASIC REFRACTORY FOREIGN PATENTS OR APPLICATIONS 1lnvemorl Barry Hieb, Pleasamon, Calif- 770,!96 10/1967 Canada ..lO6/58[73] Assignee: Kaiser Aluminum & Chemical Corporation, Oakland, CalifPrimary Examiner.lames E. Poer Att0rneyMalcolm C. McQuarrie [22] Filed:Dec. 21, 1970 [2]] App]. No.: 100,532 [57] ABSTRACT A fired periclaserefractory of enhanced density and [52] U.S. Cl ..l06/58 volumestability and good high temperature strength {51] lnt.Cl. ..C04b 35/04and thermal shock resistance is made from a batch [58] Field Of Search..l06/58, 63 ri i r latively coarse aggregate having 97,5 or

more per cent MgO and, as matrix material, a pre- [56] References cledreacted grain containing substantial amounts of lime UNITED STATESPATENTS and silica, the ratio of lime to silica being at leastsufficient to form dicalcium silicate. 3,383,226 5/l968 Hildinger..l()6/58 3,378,383 4/1968 Van Drescr 106/58 12 Claims, No DrawingsBASIC REFRACTORY BACKGROUND OF INVENTION Fired refractories wereoriginally made from a single naturally occurring substance. When suchrefractory materials are formed into shape by a nonplastic process suchas dry pressing, it is customary to use particles of varying size rangesin order to achieve maximum density in the formed shape. In makingrefractory shapes from nonplastic materials, for example magnesite orpericlase, it is customary to refer to the relatively coarse particlesof material as aggregate, and the relatively finer material as matrix.During the firing operation the matrix material sinters and coherestogether to form a ceramic bond holding the coarser aggregate together.Although the line between aggregate and matrix material is to a certainextent arbitrary, it may be considered that all material coarser than a100 mesh screen is aggregate, while all material finer than 100 mesh ismatrix.

When two different refractory materials, for example magnesite andchrome ore, are used together in a refractory composition, it is knownto have the chemical composition of the matrix differ from that of theaggregate. In the past, when a refractory was made from a singlerefractory material, for example magnesite, it was customary for thecomposition of both the coarse aggregate and fine matrix material to bethe same. In other words, the matrix material in a single componentrefractory was made by finely grinding the material used for aggregate.

However, in recent years it has come to be recognized that even in arefractory where a single component, for example magnesite or periclase,predominates, there are advantages in having the matrix material of asomewhat different composition from the aggregate. To this end, thepractice has arisen in recent years of adding fine materials to thematrixforming portion of a refractory batch. For example, lime or silicaor both may be added to periclase in a brickmaking batch in order toadjust the line to silica ratio in the matrix portion of the refractory,and more specifically to produce a refractory containing more lime andsilica in the matrix portion than in the aggregate.

SUMMARY OF THE INVENTION It has now been found that a superiorrefractory is made if, instead of adding lime and silica separately to apericlase brickmaking batch, these materials are included in the matrixportion of the batch as components of a prereacted periclase graincontaining larger amounts of lime and silica than in the overall batchcomposition. More specifically, it has now been discovered, according tothis invention, that a superior fire basic refractory can be made from abatch consisting essentially of (a) from 60 to 80 percent by weightrelatively coarse refractory aggregate, at least 90 percent of which isretained on a 100 mesh screen, said aggregate containing at least 97.5%MgO; and (b) from 20 to 40 percent by weight relatively fine matrixmaterial, at least 95% of which passes a I mesh screen, said matrixmaterial containing from 80 to 93%, by weight MgO, from 5 to CaO, andfrom 2 to 5% SiO there being at least 1.7 parts by weight CaO for eachpart by weight SiO DETAILED DESCRIPTION The coarse refractory aggregateused is a high purity periclase. Material formed by calcining a naturalmineral, for example magnesium carbonate or magnesite, of the requisitepurity can be used, but generally it will be more practical to use asynthetic periclase, for example one made from sea water or brine, toobtain the desired MgO content. The aggregate will be sized for goodbrickmaking practice, as is understood in the industry. For example, itwill all pass a 4 mesh screen and contain various size ranges down to100 mesh. A minor portion of this high purity periclase may pass a 100mesh screen. As has been mentioned, at least 90 percent of the aggregatewill be retained on a 100 mesh screen, and preferably at least 95percent will be so retained.

The aggregate will normally contain lime and silica as the predominantimpurities, and in a preferred embodiment of the invention these will bein such proportion that dicalcium silicate is the principal minor phase.It will be understood that, in addition, the aggregate may contain othernormal impurities such as alumina, iron oxide, and the like. It may alsocontain intentional additives, for example chromium oxide added to theraw material from which the grain has been made in order to promotesintering of the grain.

The refractory batch will contain a major portion of the refractoryaggregate, that is to say at least and up to 90% by weight, of the batchwill be coarse aggregate. However, most preferably the aggregate willconstitute about 70 percent, for example 65 to 75 percent, by weight ofthe batch.

The relatively fine matrix material can also be made from a naturallyoccurring material of the requisite composition, but again it willgenerally be found most practical to use a synthetic grain. The chemicalcomposition of this prereacted grain can be adjusted by adding to theraw material from which it is made, for ex ample magnesium hydroxide,the requisite amounts of lime, for example in the form of calciumcarbonate or hydroxide, and silica to the firing of this grain. Theproduct of this grain can be by any of various well known processes.

The proportions and amounts of lime and silica in the matrix grain arechosen so that the predominant secondary phase in this grain isdicalcium silicate. Thus, there will be at least 1.7 parts by weight,and preferably at least 1.87 parts by weight, CaO for each part byweight SiO It will be recognized that the weight ratio of l.87:lcorresponds to that of dicalcium silicate, containing 2 moles of CaO foreach mole of SiO As mentioned, at least 95 percent of the matrix grainwill pass a I00 mesh screen, and preferably 98 percent of it will sopass. In addition, from 60 to 75 percent of this grain will also pass a325 mesh screen. In a preferred embodiment of the invention, abouttwothirds, for example from 65 to percent by weight, will be -325 meshmaterial.

It will be understood that, in making fired refractories according tothe present invention, the batch can also contain other conventionalingredients. For example, it may contain a temporary or cold settingbond to impart strength to refractory shapes prior to firing. Also, thebatch may contain materials such as sodium nitrate or sodium citrate orcitric acid as aids to the pressing and sintering of the refractories.Finally, it will be understood that the fired shapes themselves can alsobe subjected to well known treatments, for example they may beimpregnated with tar or pitch.

1n forming refractories according to the present invention, therefractory batch, including any bonding and sintering agents desired,will be mixed with a tempering amount of water and formed into shapes,for example by pressing. Any conventional pressing means may be used,for example a de-airing press. Finally, the shapes will be fired atnormal firing temperatures for high periclase compositions, for exampleto 1700C.

An advantage of incorporating the lime and silica in the matrix by themethod of prereaction is that it avoids the problems of correctlymeasuring and adequately dispersing small amounts of these materials inthe refractory batch. In addition, the fire brick made according to thisinvention have higher densities, and better volume stability on heating,than comparable refractories made by adding lime and silica to the brickbatch.

EXAMPLE 1 A refractory batch was made from 70.2 parts by weight coarsepericlase grain, 26.6 parts by weight fine grain, 1.25 parts sodiumnitrate, 1.5 parts lignin sulfonate binder, 0.3 part dispersing agent,and 2.2 parts water. This batch, after thorough mixing, was pressed in aconventional brick press and the shapes thus formed were fired at 1600C.

Th8 coarse grain, all of which passed a 4 mesh screen and 96% of whichwas retained on a 100 mesh screen, showed the following chemicalanalysis: 0.36% SiO 0.23% Fe O 0.05% A1 0.09% Cr O 1.12% CaO, 0.19% B 0and (by difference) 97.96% MgO.

The prereacted fine grain, 98 percent of which passed a 100 mesh screenand 67 percent of which passes a 325 mesh screen, and which had aspecific surface of about 3000 cm lg, showed the following chemicalanalysis, on the fired basis: 3.47% SiO 0.34% Fe O 0.34% A1 0 0.22% Cr O6.55% CaO, 0.27% b O and (by difference) 88.81% MgO.

Bricks made from the batch of example 1 had a density of 187 lbs percubic foot (pcf) after pressing, 185 pcf after drying at 150C, and 182pcf after firing. The average volume change of four specimens uponheating from l50to 1600C was a shrinkage of 1.0 percent. The averagemodulus of rupture at 1482C of six specimens was 1934 psi.

EXAMPLE 2 A batch of 73 parts by weight coarse refractory grain, 25parts fine refractory grain, 1 part lignin sulfonate binder and 1.25parts sodium nitrate was mixed with 2.67 parts water and formed intoshapes at a pressure of 10,000 psi. These shapes were dried at 150C andfired at 1700C.

The coarse refractory grain, all of which passed a 4 mesh screen and 92percent of which was retained on a 100 mesh screen, had the followingchemical analysis: 0.4% SiO 0.2% Fe O 0.1% A1 0 1.0% CaO, 0.2% B 0 0.1%Cr O and (by difference) 98% MgO. The fine refractory grain, 98percentof which passed a 100 mesh screen, 67 percent being -325 mesh material,had the following chemical analysis: 4.0% SiO 0.5% Fe O 0.4% A1 0 7.0%CaO, 0.2% B 0 0.3% Cr O and (by difference) 87.6% MgO.

After drying at 150C, shapes made from this batch had a density of 184pcf, the density being substantially the same after firing at 1700C. Thevolumetric shrinkage upon heating to 1700C was 0.2%, and the modulus ofrupture at 1260C was 2516 psi and at 1482C, 21 1 10 psi.

From the preceding examples it can be seen that, in the preferredembodiment of the invention, the relatively fine matrix grain materialcontains from about 87.5 to 89 percent by weight MgO, from about 6.5 to7 percent by weight CaO, and from about 3.5 to 4 percent by weight SiOThe preceding example is to be compared with another made with 93 partsof the same coarse refractory grain, but sized so that the compositioncontained 23 parts passing a 100 mesh screen. This grain was admixedwith 1 part lignin sulfonate binder, 1.25 parts sodium nitrate, 3.5parts calcium carbonate, and 1.4 parts volatilized silica, the calciumcarbonate being substantially all -l00 mesh and the volatilized silicasubstantially all 325 mesh. In other words, in the comparison example,the fine prereacted grain of Example 2 was replaced with fine grain ofthe same composition as that used for the coarse aggregate, togetherwith sufficient calcium carbonate and volatilized silica to provide anequivalent amount of lime and silica. This comparison batch was mixedwith three parts water, pressed into brick shapes, and fired at 1700C.After drying at 150C, shapes from the comparison batch had a density of184 pcf, but after firing at 1700C their density was only 181 pcf. Theirvolume shrinkage on reheating to 1700C was 0.4 volume per cent, andtheir modulus of rupture at 1260C was 2886 psi, and at 1482C, 1692 psi.

The importance oflimiting the amount of-lOO mesh refractory grain ofrelatively low lime and silica content to 10 percent, and preferably to5 percent, of that grain, is shown by a case where specimens madeaccording to Example 1 inadvertently contained excess amounts of 100mesh high purity grain. In this case the density of specimens was 5 to 6pcf lower than those made according to Example 1, and the modulus ofrupture strengths were 650 psi lower.

1n the specification and claims, percentages and parts are by weightunless otherwise indicated. Mesh sizes referred to herein are Tylerstandard screen sizes which are defined in Chemical ENgineerss Handbook,John H. Perry, Editor-in-Chief, Third Edition, 1950, published by McGrawHill Book Company, at page 963. For example, a 100 mesh screen openingcorresponds to 147 microns. Analyses of mineral components are reportedin the usual manner, expressed as simple oxides, e.g., MgO, siO althoughthe components may actually be present in various combinations, e.g., asa magnesium silicate.

What is claimed is:

l. A fired basic refractory made from a batch consisting essentially of:

a. from 60 to percent by weight relatively coarse refractory aggregate,at least percent of which is retained on a 100 mesh screen, saidaggregate containing at least 97.5% MgO; and

b. from 20 to 40 percent by weight relatively fine matrix material, atleast percent of which passes a mesh screen, said matrix materialcontaining from about 87.5 to 89 percent by weight MgO,

from about 6.5 to 7 percent by weight CaO, and from about 3.5 to 4percent by weight SiO there being at least 1.7 parts by weight CaO foreach part by weight SiO 2. A refractory according to claim 1 wherein atleast 95 percent by weight of said aggregate is retained on a 100 meshscreen.

3. A refractory according to claim 1 wherein at least 98 percent of saidmatrix material passes a 100 mesh screen.

4. A refractory according to claim 1 wherein from 60 to 75 percent byweight of said matrix material passes a 325 mesh screen.

5. A refractory according to claim 4 wherein from 65 to 70 percent byweight of said matrix material passes a 325 mesh screen.

6. A refractory according to claim 1 wherein said aggregate contains atleast 1.7 parts by weight CaO for each part by weight SiO 7. Arefractory according to claim 1 containing from 65 to 75 percent byweight relatively coarse refractory aggregate and from 25 to 35 percentby weight matrix material.

8. A refractory according to claim 1 made from a refractory batchconsisting essentially of (a) about 70 percent coarse refractoryaggregate containing about 98% MgO, about 1% CaO, and about 0.4% SiO and(b) about 30 percent matrix material containing about 88% MgO, about7.5% CaO, and about 4% SiO 9. A refractory according to claim 8 whereinabout 96 percent of the aggregate is retained on a 100 mesh screen, andwherein about 98 percent of the matrix material passes a 100 mesh screenand about 67 percent of the matrix material passes a 325 mesh screen.

10. The method of making a tired refractory shape comprising:

a. preparing a coarse refractory grain containing at least 97.5% MgO,the remainder of said grain being normal impurities, at least percent ofsaid coarse grain being retained on a lOO mesh screen;

. preparing a prereacted matrix grain by firing an admixture ofMgO-yielding material with SiO yielding and CaO-yielding materials, saidprereacted grain containing, after firing, from about 87.5 to 89 percentby weight MgO, from about 6.5 to 7 percent by weight CaO, and from about3.5 to 4 percent by weight SiO there being at least 1.7 parts by weightCaO for each part by weight SiO at least percent of said matrix grainpassing a mesh screen;

. admixing from 60 to 80 percent by weight of said coarse grain withfrom 20 to 40 percent by weight of said matrix grain;

d. forming said admixture into refractory shapes; and

e. firing said shapes.

11. Method according to claim 10 wherein at least 95% by weight of saidcoarse grain is retained on a 100 mesh screen.

12. Method according to claim 10 wherein said shapes are fired at atemperature of at least l600C.

1. A fired basic refractory made from a batch consisting essentially of:a. from 60 to 80 percent by weight relatively coarse refractoryaggregate, at least 90 percent of which is retained on a 100 meshscreen, said aggregate containing at least 97.5% MgO; and b. from 20 to40 percent by weight relatively fine matrix material, at least 95percent of which passes a 100 mesh screen, said matrix materialcontaining from about 87.5 to 89 percent by weight MgO, from about 6.5to 7 percent by weight CaO, and from about 3.5 to 4 percent by weightSiO2, there being at least 1.7 parts by weight CaO for each part byweight SiO2.
 2. A refractory according to claim 1 wherein at least 95percent by weight of said aggregate is retained on a 100 mesh screen. 3.A refractory according to claim 1 wherein at least 98 percent of saidmatrix material passes a 100 mesh screen.
 4. A refractory according toclaim 1 wherein from 60 to 75 percent by weight of said matrix materialpasses a 325 mesh screen.
 5. A refractory according to claim 4 whereinfrom 65 to 70 percent by weight of said matrix material passes a 325mesh screen.
 6. A refractory according to claim 1 wherein said aggregatecontains at least 1.7 parts by weight CaO for each part by weight SiO2.7. A refractory according to claim 1 containing from 65 to 75 percent byweight relatively coarse refractory aggregate and from 25 to 35 percentby weight matrix material.
 8. A refractory according to claim 1 madefrom a refractory batch consisting essentially of (a) about 70 percentcoarse refractory aggregate containing about 98% MgO, about 1% CaO, andabout 0.4% SiO2, and (b) about 30 percent matrix material containingabout 88% MgO, about 7.5% CaO, and about 4% SiO2.
 9. A refractoryaccording to claim 8 wherein about 96 percent of the aggregate isretained on a 100 mesh screen, and wherein about 98 percent of thematrix material passes a 100 mesh screen and about 67 percent of thematrix material passes a 325 mesh screen.
 10. The method of making afired refractory shape comprising: a. preparing a coarse refractorygrain containing at least 97.5% MgO, the remainder of said grain beingnormal impurities, at least 90 percent of said coarse grain beingretained on a 100 mesh screen; b. preparing a prereacted matrix grain byfiring an admixture of MgO-yielding material with SiO2-yielding andCaO-yielding materials, said prereacted grain containing, after firing,from about 87.5 to 89 percent by weight MgO, from about 6.5 to 7 percentby weight CaO, and from about 3.5 to 4 percent by weight SiO2, therebeing at least 1.7 parts by weight CaO for each part by weight SiO2, atleast 95 percent of said matrix grain passing a 100 mesh screen; c.admixing from 60 to 80 percent by weight of said coarse grain with from20 to 40 percent by weight of said matrix grain; d. forming saidadmixture into refractory shapes; and e. firing said shapes.
 11. Methodaccording to claim 10 wherein at least 95% by weight of said coarsegrain is retained on a 100 mesh screen.